Abstract NADH:ubiquinone oxidoreductase (complex-I) is the most common site of impairment of the oxidative phosphorylation-related disorders. These disorders include some neurodevelopmental and neurodegenerative syndromes such as Leigh syndrome, MELAS, and some forms of Parkinson[unreadable]s disease. While the clinical symptoms of complex-I deficiency are defined, signaling pathways regulating complex-I function in the mitochondria are less understood. Recently, our group found that unregulated glycogen synthase kinase-3beta (GSK3beta) activity inhibits complex-I function, increases reactive oxygen species production, fragments mitochondria, and increases the cell's sensitivity to complex-I toxins. GSK3beta is a signaling protein that is known to affect metabolic pathways and brain development. Its regulation is multi-tiered meaning that its actions can be controlled by phosphorylation at its serine-9 site, by protein-protein interactions, and by its intracellular localization. Interestingly, no one has fully elucidated its intracellular distribution in the brain. In preliminary results, we have found that a salient fraction of GSK3beta exists in brain mitochondria. Furthermore, we have now found previously undiscovered pockets of GSK3beta expression in other cellular compartments. The first specific aim of this proposal is to fully determine the neuroanatomic ultrastructural distribution of GSK3beta in the mouse and human brain by electron microscopy. A thorough investigation of this sort has not been conducted previously. The actions of endogenous GSK3beta signaling in the mitochondria are not fully known. In specific aim 2, the goal is to test the hypothesis that endogenous GSK3beta modulates complex-I functions. Endogenous mitochondrial GSK3beta activity is manipulated by molecular and pharmacological methods to assess the affects of GSK3beta signaling on complex-I. The proteins STAT3 and GRIM19 are bound together in the mitochondria and both are involved in complex-I function. It is also known that GSK3beta associates with both these proteins. Our hypothesis is that GSK3beta signaling regulates complex-I activity through a tripartite protein composite consisting of GSK3beta, STAT3, and GRIM19. With this project we hope to elucidate mitochondrial GSK3beta signaling and its affects on complex-I with the prospect that our findings could be used to discover measures to alleviate complex-I disorders.